Glide Performance

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Theory of Flight

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SKYbrary

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Description

The Glide ratio of an aircraft is the distance of forward travel divided by the altitude lost in that distance. The glide ratio is affected by all of the four fundamental forces that act on an aircraft in flight - lift, drag, weight and thrust. If all these factors remain constant, the glide ratio will not change.

However, wind velocity is a very important practical influence on gliding distance over the surface. With a tailwind, the glide distance achieved will be increased because of increased groundspeed whereas with a headwind, it will be reduced because of the consequently slower groundspeed.

Variations in aircraft weight do not affect the glide angle provided that the correct airspeed is flown. Since it is the lift over drag (L/D) ratio that determines the gliding range, weight will not affect it. The glide ratio is based only on the relationship of the aerodynamic forces acting on the aircraft. The only effect weight has is to vary the time the aircraft will glide for. The heavier the aircraft is, the higher the airspeed must be to obtain the same glide ratio. If two aircraft have the same L/D ratio but different weights and start a glide from the same altitude, the heavier aircraft gliding at a higher airspeed will arrive at the same touchdown point in a shorter time. Both aircraft will cover the same distance but the lighter one will take a longer time to do so.

Drag will increase if the landing gear or flaps are extended and the airspeed will then decrease unless the pitch attitude is reduced. When pitch is reduced, the glide angle increases and the distance traveled will reduce.

The best speed for range corresponds to an angle of attack which gives the best lift-to-drag ratio. Any change in the gliding airspeed will result in a proportionate change in glide ratio. At any speed, other than the best glide speed, the glide ratio will change. When descending at a speed less than the best glide speed, induced drag increases. When descending at a speed greater than the best glide speed, parasitic drag increases. In either case, the rate of descent will increase.

Further Reading

For an account of a rare and successful glide descent carried out by a large modern transport aircraft after fuel exhaustion on a revenue transatlantic flight on 24 August 2001 - a night-time glide from FL345 to a runway 65nm away by an Airbus A330, see A332, en-route, North Atlantic Ocean, 2001